Homeotic Transformation of Ovules into Carpel-like Structures in Arabidopsis

Ovules are specialized reproductive organs that develop within the carpels of higher plants. In Arabidopsis, mutations in two genes, BELL1 (BEL1) and APETALA2 (AP2), disrupt ovule development. In Bel1 ovules, the inner integument fails to form, the outer integument develops abnormally, and the embryo sac arrests at a late stage of megagametogenesis. During later stages of ovule development, cells of the outer integument of a Bel1 ovule sometimes develop into a carpel-like structure with stigmatic papillae and second-order ovules. The frequency of carpel-like structures was highest when plants were grown under conditions that normally induced flowering and was correlated with ectopic expression in the ovule of AGAMOUS (AG), an organ-identity gene required for carpel formation. Together, these results suggested that BEL1 negatively regulates AG late in ovule development. Likewise, mutants homozygous for the strong AP2 allele ap2-6 sometimes displayed structures with carpel-like features in place of ovules. However, such abnormal Ap2 ovules are much less ovulelike in morphology and form earlier than the Bel1 carpel-like structures. Because one role of the AP2 gene is to negatively regulate AG expression early in flower development, it is possible that AP2 works in a similar manner in the ovule. A novel ovule phenotype observed in Bel1/Ap2-6 double mutants suggested that BEL1 and AP2 genes function independently during ovule development.     

A structural investigation of the ovule in sugar beet, Beta vulgaris: Integuments and micropyle

The micropyle and the integuments of sugar beet (Beta vulgaris) ovules have been investigated by light and electron microscopy during differentiation and maturation of the ovule. The micropyle itself is formed by the inner integument which is surrounded by the outer integument at its base. The micropyle containts a fibrillar PAS+ substance and is often covered by a thin sheet or hymen. Both integuments are cuticle-covered thin sheets, each 2-few cell layers in thickness. In the outer integument an increase in starch accumulation occurs during ovule maturation and probably functions as nutrient storage for embryo development. The inner epidermis of the inner integument differentiates as the most conspicuous cell layer of the beet ovule. During growth and maturation of the ovule a system of small perinuclear vacuoles containing dense material increases steadily in these cells. At maturity this system fills up more than half of each cell and very dense material has accumulated in each vacuole. This vacuole content is highly refractive and contains tannins and/or polyphenols.     

Ovule morphogenesis in Ranunculaceae and its systematic significance

BACKGROUND AND AIMS: Ranunculaceae has a prominent phylogenetic position in Ranunculales which appears at the base of eudicots. The aims of the present paper are to reveal the features of ovule morphogenesis in different taxa and gain a better understanding of the systematics of Ranunculaceae. METHODS: Flowers of 17 species from three subfamilies, nine tribes and 16 genera of Ranunculaceae, at successive developmental stages, were collected in the wild and studied with a scanning electron microscope. KEY RESULTS: The integuments in the unitegmic ovules in Helleborus, Ranunculus and Oxygraphis, as well as the inner integuments in the bitegmic genera, initiate annularly and eventually become cup-shaped. However, the integuments in the unitegmic ovules in Anemone and Clematis, as well as the outer integuments in the bitegmic genera, arise semi-annularly and eventually become hood-shaped. Different kinds of appendages appear on the ovules during development. In Coptis of subfamily Coptidoideae, a wrap-shaped appendage arises outside the ovule and envelopes the ovule entirely. In the genera of subfamily Thalictroideae and tribe Anemoneae of subfamily Ranunculoideae, appendages appear on the placenta, the funicle or both. In tribe Helleboreae of subfamily Ranunculoideae, an alary appendage is initiated where the integument and the funicle join and becomes hood-shaped. CONCLUSIONS: Ovule morphogenesis characteristics are significant in classification at the levels of subfamilies and tribes. The initiation patterns of the integuments and the development of appendages show diversity in Ranunculaceae. The present observations suggest that the bitegmic, hood-shaped outer integument and endostomic micropyle are primitive while the unitegmic, cupular-shaped outer integument and bistomic micropyle are derivative.     

Developmental morphology of the ovules of Amborella trichopoda (Amborellaceae) and Chloranthus serratus (Chloranthaceae)

The developmental morphology of the outer integument in the pendent orthotropous ovules of Amborella trichopoda (Amborellaceae) and Chloranthus serratus (Chloranthaceae) was studied. In both species the outer integument is semiannular at an early stage and becomes cup-shaped but dorsiventrally somewhat asymmetric at later stages. The outer integument, which is initiated first on the concave and lateral sides of the ovule, differs from that of the anatropous ovules of other basal families with the outer integument semiannular at an early stage or throughout development. The bilateral symmetry of the outer integument is shared by these orthotropous and anatropous ovules. The developmental pattern of the outer integument and ovule incurving characterize the ovule of the Amborellaceae and Chloranthaceae, which is not equivalent to typical orthotropous ovules of eudicots. A phylogenetic analysis of ovule characters in basal angiosperms suggests that anatropous ovules with cup-shaped outer integuments and orthotropous ovules were derived independently in several clades and that the ovules of Amborella and Chloranthus might also be derivative.     

The Arabidopsis SUPERMAN Gene Mediates Asymmetric Growth of the Outer Integument of Ovules

Arabidopsis superman (sup, also referred to as floral mutant10) mutants have previously been shown to have flowers with supernumerary stamens and reduced carpels as a result of ectopic expression of the floral homeotic gene APETALA3 (AP3). Here, we report that sup mutations also cause specific alterations in ovule development. Growth of the outer integument of wild-type ovules occurs almost exclusively on the abaxial side of the ovule, resulting in a bilaterally symmetrical hoodlike structure. In contrast, the outer integument of sup mutant ovules grows equally on all sides of the ovule, resulting in a nearly radially symmetrical tubular shape. Thus, one role of SUP is to suppress growth of the outer integument on the adaxial side of the ovule. Genetic analyses showed that the effects of sup mutations on ovule development are independent of the presence or absence of AP3 activity. Thus, SUP acts through different mechanisms in its early role in ensuring proper determination of carpel identity and in its later role in asymmetric suppression of outer integument growth.     

SHORT INTEGUMENTS 2 promotes growth during Arabidopsis reproductive development

The short integuments 2 (sin2) mutation arrests cell division during integument development of the Arabidopsis ovule and also has subtle pleiotropic effects on both sepal and pistil morphology. Genetic interactions between sin2 and other ovule mutations show that cell division, directionality of growth, and cell expansion represent at least partially independent processes during integument development. Double-mutant analyses also reveal that SIN2 shares functional redundancy with HUELLENLOS in ovule primordium outgrowth and proximal-distal patterning and with TSO1 in promotion of normal morphological development of the four whorls of primary floral organs. All of these observations are consistent with SIN2 being a promoter of growth and cell division during reproductive development, with a primary role in these processes during integument development. On the basis of the floral pleiotropic effects observed in a majority of ovule mutants, including sin2, we postulate a relationship between ovule genes and the evolutionary origin of some processes regulating flower morphology.     

The phenotype of Arabidopsis ovule mutants mimics the morphology of primitive seed plants

In seed plants, the ovule is the female reproductive structure, which surrounds and nourishes the gametophyte and embryo. This investigation describes the PRETTY FEW SEEDS2 (PFS2) locus, which regulates ovule patterning. The pfs2 mutant exhibited developmental defects in the maternal integuments and gametophyte. This mutation was inherited as a maternal trait, indicating that gametophyte defects resulted from ovule patterning aberrations. Specifically, the boundary between the chalaza and the nucellus, two regions of the ovule primordia, shifted towards the distal end of pfs2 ovule primordia. Results indicated that the PFS2 locus could: (i) be involved in the development of either the nucellus or the chalaza; or (ii) establish a boundary between these two regions. Examination of genetic interactions of the pfs2 mutation with other well-characterized ovule loci indicates that this locus affects integument morphogenesis. Interestingly, the pfs2 inner no outer and pfs2 strubbelig double mutants had inner integuments that appeared similar to their ancestral precursor. The fossil record indicates that the inner integument evolved by fusion of sterilized sporangia or branches around a central megasporangium. The question of whether the structures observed in these double mutants are homologous or merely analogous to the ancestral precursors of the inner integument is discussed.     

OVULE ABORTION IN ‘NONPAREIL’ ALMOND (PRUNUS DULCIS [MILL.] D. A. WEBB)

The earliest indication of ovule abortion in almond (Prunus dulcis [Mill.] D. A. Webb ‘Nonpareil‘) is the deposition of callose (as indicated by aniline blue fluorescence) 2 days after pollination which is 2 days before clear histological symptoms of ovule degeneration are evident and 6 days before fertilization of the viable ovule. Callose deposition begins in the chalazal region of the nucellus where the funicular trace enters the ovule and ramifies into the integuments. As ovule abortion progresses, callose deposition in the inner integument extends as a ring around the nucellus. Movement of the fluorescent dye disodium fluorescein (uranin) indicated that translocation from the vascular trace into abortive ovules becomes blocked at the chalazal position. The dye freely penetrates and diffuses into viable ovules but fails to penetrate abortive ovules. Lack of, or delayed and irregular, megagametophyte development was another characteristic of abortive ovules. Biochemical and histochemical analyses of abortive and viable ovules indicated that carbohydrate depletion parallels ovule abortion. These observations lead to the conclusion that ovule abortion is accompanied by blockage in metabolite supply although whether this blockage is the primary cause or a consequence of ovule abortion is uncertain.     

The Arabidopsis ACR4 gene plays a role in cell layer organisation during ovule integument and sepal margin development

The mechanisms regulating cell layer organisation in developing plant organs are fundamental to plant growth, but remain largely uninvestigated. We have studied the receptor kinase-encoding ARABIDOPSIS CRINKLY4 gene and shown that its expression is restricted to the L1 cell layer of most meristems and organ primordia, including those of the ovule integuments. Insertion mutations show that ARABIDOPSIS CRINKLY4 is required for regulation of cellular organisation during the development of sepal margins and ovule integument outgrowth. We show that ARABIDOPSIS CRINKLY4 encodes a functional kinase that, in ovules and possibly other tissues, is abundant in anticlinal and the inner periclinal plasma membrane of 'outside' cells. We propose that ARABIDOPSIS CRINKLY4 may be involved in maintaining L1 cell layer integrity by receiving and transmitting signals from neighbouring L1 cells and/or from underlying cell layers.     

依兰(番荔枝科)雌配子体发育研究

荔枝科是被子植物的基部类群之一,依兰属是番荔枝科较为原始的类群,其有性生殖过程,特别是胚胎发生与发育的研究结果,可以补充被子植物胚胎学原始特征的相关基础资料。该研究利用常规石蜡切片技术,对依兰胚珠、大孢子和雌配子体的发生发育过程进行了观察。结果表明:依兰的胚珠为倒生胚珠、厚珠心、三层珠被,第三层珠被(中间珠被)发生在大孢子母细胞时期,于外珠被与内珠被之间、胚珠合点端两侧发生并隆起;雌配子体为蓼型。此外,依兰的个别胚珠中存在双雌配子体现象,且两个雌配子体均由大孢子母细胞发育而来,大小、形状相近,呈线形排列。该研究结果对于揭示原始被子植物胚胎发育特征具有重要意义。     

Expression of ovule and integument‐associated genes in reduced ovules of Santalales

SUMMARY Santalales comprise mainly parasitic plants including mistletoes and sandalwoods. Bitegmic ovules similar to those found in most other angiosperms are seen in many members of the order, but other members exhibit evolutionary reductions to the unitegmic and ategmic conditions. In some mistletoes, extreme reduction has resulted in the absence of emergent ovules such that embryo sacs appear to remain embedded in placental tissues. Three santalalean representatives (Comandra, Santalum, and Phoradendron), displaying unitegmic, and ategmic ovules, were studied. Observed ovule morphologies were consistent with published reports, including Phoradendron serotinum, which we interpret as having reduced ategmic ovules, consistent with earlier reports on this species. For further understanding of the nature of the ovule reductions we isolated orthologs of the Arabidopsis genes AINTEGUMENTA (ANT) and BELL1 (BEL1), which are associated with ovule development in this species. We observed ovular expression of ANT and BEL1 in patterns largely resembling those seen in the integumented ovules of Arabidopsis. These genes were found to be expressed in the integument of unitegmic ovules and in the surface layers of ategmic ovules, and in some cases, expression of BEL1 was also observed in the surrounding carpel tissue. We hypothesize that ategmic ovules derive from a fusion of the integuments with the nucellus or that the nucellus has taken on some of the characteristics confined to integuments in ancestral species.     

Ovular development and morphology in some magnoliaceae species

Floral phenology and ovular development ofLiriodendron tulipifera are described. The ovule primordia are initiated in December, followed by prominent development in March, and the ovules are mature in May. The inner integument is formed as an annular rim on the incurving ovule primordia, but the outer integument develops as a semi-annular rim interrupted on the concave side of the funicle. Later, an outgrowth, which is interpreted here as an obturator, arises on the concave side of the funicle. The funicular outgrowth arises far from the inner integument, while the outer integument is close to the inner. The outer integument and the funicular outgrowth together form an envelope complex. Later the outer integument produces two distal lobes, which disappear at maturity. Mature ovules of the threeMagnolia species examined have similar lobes. It is suggested that the hood-shaped outer integument is primitive in angiosperms.     

The histogenesis of integuments in some species of menispermanceae

The mode of initiation and development of integuments was investigated in six species of five genera in Menispermanceae, which have bitegmic and unitegmic ovules. The species investigated have similar integumentary structures at maturity in each of the bitegmic and unitegmic ovules. In bitegmic ovules (e.g.Cocculus), both integuments are for the most part two-cell layered. The initiation of inner integument (ii) begins with divisions of dermal cells of the nucellar primordium. The initiation of the outer integument (oi) commences with divisions of subdermal cells. In unitegmic ovules (e.g.Stephania), the integument is initiated by periclinal divisions of dermal cells, and cells of subdermal origin (which may represent the oi in case of bitegmy) form a small swelling on the raphal side and, on the antiraphal side, are included in the base of the single integument. Unitegmy of Menispermanceae (at least in the case of the genera investigated) seems to have been derived through elimination of oi, rather than through “integumentary shifting” (Bouman and Calis, 1977), a process suggested for explanation of unitegmy as in Ranunculaceae.